507
The results of many investigations led to the conclusion that the pathogen consists of a phenotypically and serologically homogeneous taxon (e.g. Magariños
et al. 1992 ) but is genetically heterogeneous as determined by results of substractive
hybridisation (Juíz-Río et al. 2005 ). By ribotyping of 29 isolates, 2 major ribotypes
were recognised which effectively separated European and Japanese isolates. A
third ribotype accommodated a unique strain (Magariños et al. 1997b ).
The morphology and physiology of this pathogen led Snieszko et al. ( 1964 ) to
suspect a similarity to the genus Pasteurella. This view was reinforced by crossprecipitin reactions with Pasteurella (Yersinia) pestis. From this deduction, Janssen
and Surgalla ( 1968 ) realised that, from an examination of 27 isolates, the organism
was very homogeneous and different from existing species of Pasteurella. Therefore,
the name of Pa. piscicida was coined. Independently, Kusuda gave it an alternative
name, i.e. Pa. seriola, but quickly realised its synonymy with Pa. piscicida, which
was accorded preference. However, Pa. piscicida was not included in the ‘Approved
Lists of Bacterial Names’ (Skerman et al. 1980 ) or their supplements. Consequently,
the name of Pa. piscicida lacked taxonomic validity.
A detailed taxonomic evaluation based on small-subunit rRNA sequencing and
DNA: DNA hybridisation revealed that the organism was highly related to Ph. damselae (there was >80 % relatedness of the DNA) , and it was proposed that the organism be accommodated in a new subspecies, as Ph. damsela subsp. piscicida
(Gauthier et al. 1995 ) , the epithet of which was corrected to damselae (Trüper and
De’Clari 1997 ), as Ph. damselae subsp. piscicida. AFLP analysis revealed that the
two subspecies are indeed distinct and separate entities (Thyssen et al. 2000 ).
To complicate matters, there is controversy over interpretation of the Gramstaining reaction. The majority opinion is that the organism is Gram-negative .
However, Simidu and Egusa ( 1972 ) considered that cells displayed Gram-variability
when young, i.e. in 12–18 h cultures incubated at 20–25 °C. In addition, they presented photographic evidence which showed that cells shortened with age. In fact,
the suggestion was made that the pathogen is related to Arthrobacter. It is ironic that
a similar phenomenon, concerning the interpretation of Gram-stained smears, was
reported by Kilian ( 1976 ) and Broom and Sneath ( 1981 ) for Haemophilus piscium,
the causal agent of ulcer disease.
Diagnosis
Phenotypic Methods Kent ( 1982 ) reported that Ph. damselae subsp . piscicida
gave positive responses in the API 20E rapid identifi cation system for arginine dihydrolase and weak acid production from glucose; all other tests were negative. Of
course, it is necessary to modify the protocol for use with marine bacteria. Thus, it
was essential to suspend cultures in 2–3 % (w/v) saline rather than distilled water,
and the inoculated test strips were incubated at 25 °C (not 37 °C) for up to 48 h.
Photobacterium damselae subsp. piscicida (= Pasteurella piscicida)
The results of many investigations led to the conclusion that the pathogen consists of a phenotypically and serologically homogeneous taxon (e.g. Magariños
et al. 1992 ) but is genetically heterogeneous as determined by results of substractive
hybridisation (Juíz-Río et al. 2005 ). By ribotyping of 29 isolates, 2 major ribotypes
were recognised which effectively separated European and Japanese isolates. A
third ribotype accommodated a unique strain (Magariños et al. 1997b ).
The morphology and physiology of this pathogen led Snieszko et al. ( 1964 ) to
suspect a similarity to the genus Pasteurella. This view was reinforced by crossprecipitin reactions with Pasteurella (Yersinia) pestis. From this deduction, Janssen
and Surgalla ( 1968 ) realised that, from an examination of 27 isolates, the organism
was very homogeneous and different from existing species of Pasteurella. Therefore,
the name of Pa. piscicida was coined. Independently, Kusuda gave it an alternative
name, i.e. Pa. seriola, but quickly realised its synonymy with Pa. piscicida, which
was accorded preference. However, Pa. piscicida was not included in the ‘Approved
Lists of Bacterial Names’ (Skerman et al. 1980 ) or their supplements. Consequently,
the name of Pa. piscicida lacked taxonomic validity.
A detailed taxonomic evaluation based on small-subunit rRNA sequencing and
DNA: DNA hybridisation revealed that the organism was highly related to Ph. damselae (there was >80 % relatedness of the DNA) , and it was proposed that the organism be accommodated in a new subspecies, as Ph. damsela subsp. piscicida
(Gauthier et al. 1995 ) , the epithet of which was corrected to damselae (Trüper and
De’Clari 1997 ), as Ph. damselae subsp. piscicida. AFLP analysis revealed that the
two subspecies are indeed distinct and separate entities (Thyssen et al. 2000 ).
To complicate matters, there is controversy over interpretation of the Gramstaining reaction. The majority opinion is that the organism is Gram-negative .
However, Simidu and Egusa ( 1972 ) considered that cells displayed Gram-variability
when young, i.e. in 12–18 h cultures incubated at 20–25 °C. In addition, they presented photographic evidence which showed that cells shortened with age. In fact,
the suggestion was made that the pathogen is related to Arthrobacter. It is ironic that
a similar phenomenon, concerning the interpretation of Gram-stained smears, was
reported by Kilian ( 1976 ) and Broom and Sneath ( 1981 ) for Haemophilus piscium,
the causal agent of ulcer disease.
Diagnosis
Phenotypic Methods Kent ( 1982 ) reported that Ph. damselae subsp . piscicida
gave positive responses in the API 20E rapid identifi cation system for arginine dihydrolase and weak acid production from glucose; all other tests were negative. Of
course, it is necessary to modify the protocol for use with marine bacteria. Thus, it
was essential to suspend cultures in 2–3 % (w/v) saline rather than distilled water,
and the inoculated test strips were incubated at 25 °C (not 37 °C) for up to 48 h.
Photobacterium damselae subsp. piscicida (= Pasteurella piscicida)